A number of substances of commercial significance may be produced from natural sources, including biomass. Cellulosic biomass may be particularly advantageous in this regard due to the versatility of the abundant carbohydrates found therein in various forms. As used herein, the term “cellulosic biomass” refers to a living or recently living biological material that contains cellulose. The lignocellulosic material found in the cell walls of higher plants is the world's largest source of carbohydrates. Materials commonly produced from cellulosic biomass may include, for example, paper and pulpwood via partial digestion, and bioethanol by fermentation.
Significant attention has been placed on developing fossil fuel alternatives derived from renewable resources. Cellulosic biomass has garnered particular attention in this regard due to its abundance and the utility of its constituent carbohydrates. Despite promise and intense interest, the development and implementation of bio-based fuel technology has been slow. Existing technologies have heretofore produced fuels having a low energy density (e.g., bioethanol) and/or that are not fully compatible with existing engine designs and transportation infrastructure (e.g., methanol, biodiesel, Fischer-Tropsch diesel, hydrogen, and methane). An energy- and cost-efficient process for processing cellulosic biomass into fuel blends having similar compositions to fossil fuels would be highly desirable to address the foregoing issues.
Digestion is one way in which the complex carbohydrates within cellulosic biomass can be converted into a form more suitable for being processed into fuel blends. Specifically, digestion can break down the complex carbohydrates within cellulosic biomass into a hydrolysate containing simpler, soluble carbohydrates, which may be further transformed into oxygenated organic compounds through downstream reforming reactions. Although the underlying chemistry behind digesting complex carbohydrates and further transforming simple carbohydrates into organic compounds reminiscent of those present in fossil fuels is understood, high-yield and energy-efficient digestion processes suitable for use in converting cellulosic biomass into fuel blends have yet to be developed. In this regard, the most basic requirement associated with converting cellulosic biomass into fuel blends using digestion and other processes is that the energy input needed to bring about the conversion should not be greater than the available energy output of the product fuel blends. Furthermore, the amounts of the fuel blends produced per unit mass of cellulosic biomass feed should ideally be as high as possible.
The issues associated with digesting cellulosic biomass and converting the hydrolysate into fuel blends in high yields and in an energy- and cost-efficient manner are exceedingly complex, as further discussed hereinafter. In addition, these issues are entirely different than those encountered in the digestion processes commonly used in the paper and pulpwood industry. Since the intent of cellulosic biomass digestion in the paper and pulpwood industry is to retain a solid material (e.g., wood pulp), incomplete digestion is usually performed at low temperatures (e.g., less than about 100° C.) for a fairly short period of time. In contrast, digestion processes suitable for converting cellulosic biomass into fuel blends are ideally configured to maximize yields by solubilizing as much of the original cellulosic biomass charge as possible.
Production of greater quantities of soluble carbohydrates for use in fuel blends via routine modification of paper and pulpwood digestion processes is not feasible for a number of reasons. Simply running the digestion processes of the paper and pulpwood industry for a longer period of time to produce more soluble carbohydrates is undesirable from a throughput standpoint. Use of digestion promoters such as strong alkalis, strong acids, or sulfites to accelerate the digestion rate can increase process costs and complexity due to post-processing separation steps and the possible need to protect downstream components from these agents. Accelerating the digestion rate by increasing the digestion temperature can actually reduce yields due to thermal degradation of soluble carbohydrates that can occur at elevated digestion temperatures. Use of higher digestion temperatures can also be undesirable from an energy efficiency standpoint. In addition, extended digestion times under elevated temperature and pressure conditions may sometimes lead to structural failure issues of the digestion unit if not compensated for in some manner. Any of these difficulties can defeat the economic viability of fuel blends derived from cellulosic biomass.
One way in which soluble carbohydrates within a hydrolysate can be protected from thermal degradation is through subjecting them to a catalytic reduction reaction process, which may include hydrogenation and/or hydrogenolysis reactions. The products of such catalytic reduction reaction processes may be readily transformable into fuel blends through downstream reforming reactions. Stabilizing soluble carbohydrates within a hydrolysate through conducting a catalytic reduction reaction process may allow digestion of cellulosic biomass to take place at higher temperatures without unduly sacrificing yields.
One way in which a hydrolysate derived from cellulosic biomass may be stabilized very efficiently is through conducting a catalytic reduction reaction process concurrently with the digestion process, ideally in the same vessel in which the digestion process takes place. A catalytic reduction reaction process that occurs in the same vessel as a digestion process will be referred to herein as an “in situ catalytic reduction reaction process.” In addition to stabilization of the hydrolysate, conducting an in situ catalytic reduction reaction process may be very favorable from an energy efficiency standpoint. Specifically, the digestion of cellulosic biomass is an endothermic process, whereas catalytic reduction reactions are exothermic. Thus, the excess heat generated by the catalytic reduction reaction process may be utilized to drive the digestion process, thereby lowering the amount of additional energy input needed to conduct digestion. Since digestion and catalytic reduction take place together in an in situ catalytic reduction reaction process, there is minimal opportunity for heat transfer loss to take place, as would occur if the catalytic reduction reaction process were to be conducted in a separate location. In addition, conducting the catalytic reduction reaction process in a separate location may increase the risk of degradation of soluble carbohydrates occurring during transit.
Although conducting an in situ catalytic reduction reaction process may be particularly advantageous for purposes of hydrolysate stabilization and energy efficiency, successfully executing such a coupled process may be problematic in other aspects. One significant issue that may be encountered is that of catalyst distribution within the digesting cellulosic biomass charge. Without sufficient catalyst distribution being present, ineffective stabilization of soluble carbohydrates may occur. In addition, maintaining fluid flow within the digestion unit can be another significant issue. The embodiments presented herein address many of the foregoing issues as well as providing related advantages.